Conventional magnetic disk drive designs comprise a commonly denominated Contact Start-Stop (CSS) system commencing when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface and is supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions to allow data to be recorded on and retrieved from the surface of the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from a stop and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, and sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
It is considered desirable during reading and recording operations to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk to be positioned in close proximity with an attendant increase in predictability and consistent behavior of the air bearing supporting the head. However, if the head surface and the recording surface are too flat, then the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces eventually leading to what is referred to as a "head crash." Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing these apparent competing objectives involve providing a magnetic disk with a roughened surface to reduce the head/disk friction by techniques generally referred to as "texturing." Conventional texturing techniques involve polishing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated in the subsequently deposited layers.
The escalating requirements for high areal recording density impose increasingly greater requirements on thin film magnetic media in terms of coercivity, stiction, squareness, low medium noise and narrow track recording performance. In addition, increasingly high density and large-capacity magnetic disks require increasingly smaller flying heights, i.e., the distance by which the head floats above the surface of the disk in the CSS drive. The requirement to further reduce the flying height of the head challenges the limitations of conventional technology for controlled texturing to avoid head crash.
Conventional techniques for providing a disk substrate with a textured surface comprise a mechanical operation, such as polishing. In texturing a substrate for a magnetic recording medium, conventional practices comprise mechanically polishing the surface to provide a data zone having a substantially smooth surface and a landing zone characterized by topographical features, such as protrusions and depressions. See, for example, Nakamura et al., U.S. Pat. No. 5,202,810. Conventional mechanical texturing techniques, however, are attendant with numerous disadvantages. For example, it is extremely difficult to provide a clean textured surface due to debris formed by mechanical abrasions. Moreover, the surface inevitably becomes scratched during mechanical operations, which contributes to poor glide characteristics and higher defects. Such relatively crude mechanical polishing with attendant scratches and debris makes it difficult to obtain adequate data zone substrate polishing for proper crystallographic orientation of a subsequently deposited magnetic layer. In addition, various desirable substrates are difficult to process by mechanical texturing. This undesirably limiting facet of mechanical texturing, virtually excludes the use of many materials for use as substrates.
Some important parameters of a media surface that affect performance are the roughness level (Ra) and the glide avalanche. The roughness level is a measure of the average roughness of the surface and the glide avalanche is a measure of how close to the surface a flying head can fly. FIG. 1 depicts an exemplary glide testing profile of a media surface that has been textured by mechanical polishing techniques to a roughness level of Ra=7.2 .ANG.. The glide avalanche of such a surface based on this testing is below 0.6 .mu.(at about 0.55 .mu.").
The flying performance of a Winchester-type slider is primarily affected by the micro-waviness that is close to the low frequency portion of the roughness profile. A low frequency profile of the exemplary surface profile of FIG. 1 is depicted in FIG. 2. The bumps created by the mechanical texturing vary greatly in size over a range of 200 .mu.m in the radial direction of the disk. The largest bump in FIG. 2 is 50 .ANG. high, and the other bumps have lesser heights. The surface profile is thus a relatively random profile, with no specified number of peaks, nor defined heights of the bumps and depths of the valleys. Since the dominant factors affecting the glide avalanche are the low frequency profile and the maximum overall height of the surface, the randomness of the low frequency profile created by mechanical polishing leads to an unpredictable glide avalanche and an unpredictable flying head performance. This is so even though the overall average roughness is 7.2 .ANG., since this parameter represents an average and does not necessarily reflect the extent and number of peaks and valleys on the surface.
An alternative technique to mechanical texturing for texturing a landing zone comprises the use of a laser light beam focused on an upper surface of a non-magnetic substrate. See, for example, Ranjan et al., U.S. Pat. No. 5,062,021, wherein the disclosed method comprises polishing an NiP plated A1 substrate to a specular finish, and then rotating the disk while directing pulsed laser energy over a limited portion of the radius, to provide a textured landing zone leaving the data zone specular. The landing zone comprises a plurality of individual laser spots characterized by a central depression surrounded by a substantially circular raised rim.
Another laser texturing technique is reported by Baumgart et al. "A New Laser Texturing Technique for High Performance Magnetic Disk Drives," IEEE Transactions on Magnetics, Vol. 31, No. 6, pp. 2946-2951, November 1995. See, also, U.S. Pat. Nos. 5,550,696 and 5,595,791.
In copending application Ser. No. 08/666,374 filed on Jun. 27, 1996, now U.S. Pat. No. 5,968,608, a laser texturing technique is disclosed employing a multiple lens focusing system for improved control of the resulting topographical texture. In copending application Ser. No. 08/647,407 filed on May 9, 1996, now U.S. Pat. No. 5,783,797, a laser texturing technique is disclosed wherein a pulsed, focused laser light beam is passed through a crystal material to control the spacing between resulting protrusions.
In copending U.S. Pat. No. 5,955,154, a method is disclosed for laser texturing a glass or glass-ceramic substrate employing a laser light beam derived from a CO.sub.2 laser source. The textured glass or glass-ceramic substrate surface comprises a plurality of protrusions which extend above the substrate surface, without surrounding valleys extending substantially into the substrate as is characteristic of a laser textured metallic substrate. The effect of laser parameters, such as pulse width, spot size and pulse energy, and substrate composition on the protrusion or bump height of a laser textured glass or glass-ceramic substrate is reported by Kuo et al., in an article entitle "Laser Zone Texturing on Glass and Glass-Ceramic Substrates," presented at The Magnetic Recording Conference (TMRC), Santa Clam, Calif., Aug. 19-21, 1996.
In copending application Ser. No. 08/796,830 filed on Feb. 7, 1997, now U.S. Pat. No. 5,714,207, a method is disclosed for laser texturing a glass or glass-ceramic substrate, wherein tie height of the protrusions is controlled by controlling the quench rate during resolidification of the laser formed protrusions. One of the disclosed techniques for controlling the quench rate comprises preheating a substrate, as by exposure to a first laser light beam, and then exposing the heated substrate to a focused laser light beam.
Although laser texturing techniques have proven advantageous, they have not been hitherto employed to texture an ultra-fine pattern on a media surface to have an ultra-low asperity height while ensuring overall tribological glide performance. In order to ensure the tribological glide performance, uniform wave heights represented by elongated aspirates are needed.